Law enforcement officers search the campus of Santa Monica College following a shooting at the school in Santa Monica, California, June 7, 2013.
Reuters

John Zawahri, the Santa Monica shooter who went on a rampage on June 7 gunning down five people, left a farewell letter regretting the killing of his father and brother, police said on Thursday.

The 23-year-old also bid goodbye to his friends and expressed hope his mother would be cared for, Associated Press reported. The content of the note is yet to be made public.

The note was found on Zawahri’s body after police shot and killed him on June 7 on the Santa Monica College campus, police chief Jacqueline Seabrooks told a news conference, according to AP.

She noted that the reason behind Zawahri’s actions is yet unknown, but investigators believe mental illness played a key role. The shooter is also said to have struggled with his parents' divorce.

"We know his was a troubled life and that he experienced mental health challenges," Seabrooks said. "We believe that his mental health challenges likely played a role in his decisions to shoot and kill both his father and his brother, to set fire to the family home, and to go on a 13-minute shooting spree spanning roughly 1.5 miles and which left five innocent people dead and three people injured."

Reports indicate that Zawahri’s childhood was marred with trauma and violence, and that he was once admitted to UCLA’s psychiatric ward while studying at Olympic High.

An English teacher once saw him surfing the Internet for assault weapons and he was promptly sent to the principal’s office, the teacher, who wanted to remain anonymous, told the Los Angeles Times.

Within days, the police were involved and Zawahri was admitted to UCLA’s psychiatric unit. But he did not remain in the facility for long.

"We all said, why in hell did they let him out? But they had their legal reasons," the teacher told the Times.

Another teacher, Wendy Parise, remembered encountering Zawahri and his mother Randa Abdou during a special education program at a Santa Monica pre-school in 1993, soon after his parents filed for divorce.

"He was very withdrawn and clingy to his mother," Parise told the Times. "It was obvious to me there was trauma in the home."